1. Field of the Invention
The present invention relates generally to electro-optically switched directional couplers, and, more particularly, to an electro-optically switched directional coupler utilizing reverse differential propagation constant (.DELTA..beta.) control which has improved polarization-independent and operating voltage tolerant characteristics.
2. Background of the Invention
Electro-optically switched directional couplers are typically used in optical devices and systems to switch light signals among wave guides that are optically coupled together in an electro-optic material. In a typical application, a 2.times.2 electro-optically switched directional coupler would be connected, for example, to a first pair of optical fibers at input ports and a second pair of optical fibers at output ports. A pair of wave guides defined in the electro-optic material transmit light signals from the input ports to the output ports through a coupling region where the light signals may either cross between channels or stay in the same channel. For example, a light signal entering an input port for a first wave guide can be transferred within the 2.times.2 directional coupler so as to exit at an output port for a second wave guide, in which case the directional coupler is said to be in a "cross state". Alternatively, the light signal entering the input port for the first wave guide can be passed through the 2.times.2 directional coupler so as to remain in the first wave guide and exit at an output port for the first wave guide, in which case the directional coupler is said to be in the "bar" state. Because light signals may be passed both ways through a 2.times.2 directional coupler, light signals might also "enter" the output ports and "exit" the input ports.
Ideally, the directional coupler is designed so as to electro-optically switch the light signals between the cross state and the bar state in response to an electric field that is applied across the coupling region within the directional coupler. The coupling region is located within the directional coupler in a region where the wave guide channels are very close to one another. In this region, the wave guide channels are constructed so as not to restrict the light signal to staying within a particular channel. Consequently, a light signal passing through the coupling region is free to stay within one wave guide channel, cross over to the other wave guide channel, or do both, depending upon the optical transmission properties of the wave guide channels within the coupling region. When an electric field is applied across the coupling region of an electro-optic material, the electric field can change the optical transmission properties of the wave guide channels. As a result, the way in which the light signal passes through the coupling region can also be changed.
In a conventional electro-optically switched directional coupler, two single mode wave guide channels are fabricated in an electro-optically active substrate by increasing the effective refractive index of the substrate in an area corresponding to the wave guide channels. In a coupling region where the wave guide channels are physically close together, the evanescent optical fields of the two channels overlap, resulting in an optical coupling between the wave guide channels. The effect of coupling between the wave guide channels can be selectively controlled by electro-optically modifying the refractive index in the coupling region via an electric field produced by two electrode pairs situated in the coupling region near the wave guide channels. The use of two pairs of electrodes in this manner is referred to as a directional coupler utilizing reverse differential propagation constant (.DELTA..beta.) control. Two electrode pairs, or alternatively two independent control electrodes and a single common electrode, are used to electro-optically modify the refractive index of the coupling region, rather than a single electrode pair, in order to obtain better control of the switch state of the directional coupler.
While it would be desirable if the directional coupler behaved exactly like a digital switch that was always either completely on (the bar state) or completely off (the cross state), the control of the light signal as it passes through the coupler is not that simple. In practice, an electro-optically switched directional coupler behaves more like a leaky two-way valve, with most of the light signal being transferred through the desired wave guide channel, but with some of the light signal leaking out the other wave guide channel. As long as the relative difference between the optical outputs of each wave guide channel is large enough, however, it is still possible to use the coupler as an effective optical switch. When this relative difference between the optical output power of each wave guide channel is expressed as a ratio, it is referred to as the switching extinction ratio of the directional coupler. By convention, the extinction ratio of a reverse .DELTA..beta. directional coupler is normally defined to be equal to 10 log.sub.10 (c/b), where b is the output power of the bar port and c is the output power of the cross port. When the directional coupler is behaving in the desired manner of a digital switch, the extinction ratio will typically be greater than 15 dB, meaning that less than about 1/30th of the light signal will be emerging from the non-intended port of the directional coupler. To operate a reverse .DELTA..beta. control directional coupler in the desired manner like a digital switch, there will be a cross state set of voltages V.sub.1c and V.sub.2c which represents the operating voltages that should be applied to the pair of electrodes in order to induce a particular directional coupler to operate in the cross state. In addition, there will be a bar state set of voltages V.sub.1b and V.sub.2b which correspond to the operating voltages that should be applied to the pair of electrodes in order to induce the same directional coupler to operate in the bar state.
With this background in mind, it is possible to discuss the present state of efforts to enhance the operation and utility of reverse .DELTA..beta. control directional couplers. These efforts generally fall into one of four categories: (1) efforts to increase the switching extinction ratio of the device; (2) efforts to decrease or shift the operating voltages required to operate the device; (3) efforts to decrease the sensitivity of the device to normal variations in operating conditions; and (4) efforts to decrease the sensitivity of the device to variations in the fabrication process. While the present invention is directed to the later two of these four categories, it is helpful to examine the nature of previous work in the other two categories to understand why this work is not necessarily related to the objectives of the present invention.
A variety of design approaches to increase the switching extinction ratio of directional couplers are described in U.S. Pat. No. 5,255,334, issued to Mak et al., including the creation of additional partial wave guides between the wave guide channels, the use of additional electrodes outside of the coupling region, and the creation of additional partial junctions within wave guide channels. In Electronics Letters 23, No. 21, 8th October 1987, p. 1145, Okayama et al. describe a directional coupler which uses wave guide channels having differential widths in order to provide a built-in asymmetry between the wave guide channels that decreases the operating voltages of the directional coupler. U.K. Patent 2 223 323 issued to Walker describes a curved directional coupler that also seeks to provide a built-in asymmetry by using differential radii of the wave guide channels to move the relative location of the operating voltages of the device. Japanese patent HEI 1[1989]-243037 issued to Ohta describes an S-shaped curve having a smoothly varying curvature for the wave guide channels that seeks to achieve both of these advantages.
One common problem with all of these directional couplers is that the various designs effectively ignore the dependence of the coupler upon the polarization of the light signals passing through the coupler. With the possible exception of the Ohta patent, each of these references assumes "ideal or controlled operating conditions" for a single directional coupler designed in accordance with these references. In particular, none of these references deal with the case of an optical signal of uncontrolled polarization.
In addition, none of these references address the issue of "inter-device variations" due to the sensitivity of the device to variations in the fabrication process. As used within the present invention, operating conditions will refer to the environmental and control conditions under which the device is to be used, including, for example, the sensitivity of the device to variations in temperature, humidity, operating voltage drift, and input light signal conditions, including the polarization of the input light signals. In practice, the performance of a group of similar electro-optic devices is determined not only by the sensitivity of a given device to the operating conditions, but also by the inter-device variation cause by unavoidable differences in their manufacture.
In a real world application outside of a laboratory environment, the lack of ideal or controlled operating conditions further complicates the problems of inter-device variation when the inter-device variation is large for a group of electro-optic couplers. If the variation from device-to-device is so large as to require individual tuning of each device to insure proper operation, then there can be no mass production or interchangeability of the devices outside the laboratory environment. In real world operating conditions, these issues have resulted in two major problems which currently limit the widespread use of electro-optically switched directional couplers outside a laboratory environment: (1) the lack of polarization independence; and (2) the lack of inter-device uniformity of operating voltage.
Polarization independence refers to the ability of an optical device to perform its intended function regardless of the polarization of the light signal. The polarization of a light signal is defined as the orientation of the electric field components that comprise the light signal as the light signal is electromagnetically propagated through a wave guide. A transverse electric (TE) polarization is defined as when the electric field comprising the light signal is parallel to the surface of the substrate or channel through which the light signal propagates. A transverse magnetic (TM) polarization is defined as when the electric field produced by the light signal is perpendicular to the surface of the substrate or channel.
In most practical applications, the light signals within a directional coupler will be in an uncontrolled polarization state. As result, it is desirable for the optical device to be polarization independent in that the device is designed to function equally as well in any polarization state. Unfortunately, it is very difficult to make a directional coupler polarization independent. Many optical devices which use existing directional couplers have simply ignored the problem of polarization independence. While these devices will function as expected under the right conditions, there will be conditions in which the device will not behave as expected. Most optical devices which make use of existing directional couplers avoid this problem altogether by using light signals of controlled polarization to insure that all of the light passing through the coupler is polarized in the same orientation.
For those directional couplers which have attempted to achieve polarization independence, one common approach is to carefully control the geometries of the directional coupler by selecting the length (L) of the coupling region and/or the spacing (G) between the wave guide channels in the coupling region so as to control an effective coupling length (l) that will contrive the bar and cross states to be consistent regardless of the polarization of the light signal. An alternate approach to achieving polarization independence is described in U.S. Pat. No. 4,243,295 issued to Alferness which employs a spatial tapering of the spacing (G) between the centers of wave guides so as to preserve the TE and TM bar state while allowing independent choice of coupling to achieve the cross state. Still another approach to polarization independence utilizing a four section optical coupler with additional electrodes and control circuitry is described in U.S. Pat. No. 5,202,941 issued to Granestand.
Unfortunately, none of these techniques provides for an optimum solution to the problem of designing polarization independence into a directional coupler. In the conventional approach, the manufacturer of the directional coupler must sacrifice the ability to freely choose the length L and spacing G which might otherwise be varied to achieve other design objectives. While the tapered coupler described by Alferness improves somewhat on the restrictions of the conventional approach, the same types of limitations still exist with respect to the freedom of choice of lengths L and spacings G which are actually useable. In the four section optical coupler described by Granestand, additional electrodes and control circuitry significantly increase the complexity of both fabrication and control of the device.
Inter-device operating voltage tolerance refers to the tolerance range of the operating voltages V.sub.1 and V.sub.2 to maintaining a given switch state for a group of similar directional couplers. The tolerance range of the operating voltages is determined mainly by two factors. First, variations in operating voltage characteristics can be caused by process variations in the manufacturing of different ones of the directional couplers. Second, variations in operating voltage characteristics will occur with variations in environmental conditions, such as temperature, humidity, electric field exposure, age and the like.
Larger operating voltage tolerances are beneficial in at least three ways. First, larger operating voltage tolerances improve manufacturing yield. The improvement in manufacturing yield can be understood by noting that polarization independence requires a coincidence of the operating voltages for the bar states and cross states for the two principal polarization modes, something which requires accurate control of the coupling characteristics of the wave guides. Because the coupling characteristics of a wave guide are very dependent on the details of the manufacturing process, small fluctuations in the manufacturing process conditions can cause the coupling characteristics to be slightly different than the intended target characteristics required for coincidence. As a result, the operating voltages of the bar states and cross states may not be coincident and the device would fail to meet operational specifications, in which case the time, labor and raw materials to generate the device will have been wasted. Second, larger operating voltage tolerances extend the range of environmental conditions under which a given device will operate. As the operating conditions vary, particularly ambient environmental conditions such as temperature or wavelength, the operating voltages required for correct operation of the two principal polarization modes will change, usually by different amounts for each polarization mode. If these changes cause the operating voltages for the bar states and cross states for the different polarizations to go out of coincidence, the device will cease to be polarization independent. Finally, larger operating voltage tolerances reduce the need for monitoring the switch state in order to provide feedback control for the operating voltages. This reduction in the need for feedback control simplifies the use of the device under different operating conditions, including compensating for changes as the device ages. As a result, the device is easier and less expensive to make and use, and the device can be used in a wider range of applications.
While there is a growing interest in the use of reverse .DELTA..beta. control directional couplers for optical devices and systems, the current limitations with regard to polarization independence and operating voltage tolerances have effectively limited the use of these devices to laboratory environments or those situations where the cost of artificially simulating laboratory conditions through polarization control and individual tuning of the voltages to each switch is not prohibitive. Consequently, there is a need for an improved reverse .DELTA..beta. control directional couplers which can overcome the present limitations with regard to polarization independence and inter-device operating voltage tolerances of these devices.